The present invention relates to a new and improved construction of a self-monitoring warning or alarm installation containing a monitoring circuit and an alarm or warning circuit.
Alarm installations are used in those environments where there is monitored a variable magnitude or parameter and when there should be indicated that such variable magnitude has fallen below or exceeded a predetermined threshold. Typical examples are installations for monitoring the terminal position of displaceable machine components, the temperature or the concentration of combustion gases in rooms, the degree of filling of vessels or containers, the subjecting to jarring or damage of building openings or the deformation or displacement of terrain formations and structural walls.
A first group of heretofore known warning or alarm installations contains passive feelers which are affected by the magnitude or parameter which is to be monitored. These feelers are connected by means of an electrical signal line with an electronic monitoring circuit. This monitoring circuit periodically interrogates the state of the feeler and produces an alarm signal when this state or condition no longer is within a predetermined region or range.
A second group of prior art installations contain active feelers which are equipped with a related energy source or are connected with a central energy source. These feelers generate a signal as soon as the monitored magnitude has fallen below or exceeded a predetermined threshold. This signal again is transmitted by means of an electrical signal line to a central monitoring circuit. With a number of design constructions of such installations there is used the same electrical line or conductor for the infeed of the electrical energy to the feeler and for the return of the monitoring signal to the monitoring circuit. Alarm or warning installations are preferably structured as self-monitoring installations, generating a display or indicator signal when parts of the installation no longer are functional.
The heretofore known installations possess, independently of the special constructional embodiments and fields of applications, a number of basic defects. The cause of such defects or flaws are the use of feelers or sensors which are electrically interrogated or themselves generate electrical signals and the electrical signal lines needed for the interrogation and further transmission of the signals. Spurious signals can be induced in such feelers and the lines due to external electrical or magnetic fields. These spurious signals impair the proper functioning of the installation or can trip false alarms. In order to eliminate these defects it is necessary to electrostatically screen the feelers and lines. The procedure is expensive and not possible for many feelers without impairing their function. Moreover, electrical feelers or sensors and signal lines are not permissible in an explosion-endangered environment, although it is exactly at such locations that there is particularly needed the monitoring of different operating magnitudes.